Print cause of exceptions even when they have no traceback

[nilsvu]

Exceptions from subprocesses raised by 'multiprocessing' don't include a traceback, so rich doesn't currently print them. They contain important information though, so this PR enables printing the exception without the traceback. This works because walk_tb doesn't do anything if the input is None.

Minimal example:

import rich.traceback
import multiprocessing


def fail():
    raise ValueError


if __name__ == "__main__":
    rich.traceback.install()
    with multiprocessing.Pool() as pool:
        # The traceback printed by rich doesn't point to the function `fail`,
        # but only to code in the `multiprocessing` module. This PR fixes that.
        pool.apply(fail)

Note that exceptions from multiprocessing include the traceback formatted as a string in the description of the exception (see this code in multiprocessing.pool: https://github.com/python/cpython/blob/main/Lib/multiprocessing/pool.py#L57). This will be printed by rich after this PR.

Type of changes

• [x] Bug fix
• [ ] New feature
• [ ] Documentation / docstrings
• [ ] Tests
• [ ] Other

Checklist

• [x] I've run the latest black with default args on new code.
• [x] I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• [x] I've added tests for new code.
• [x] I accept that @willmcgugan may be pedantic in the code review.

[willmcgugan]

Can I see an example of how multiprocessing exceptions are printed?

[nilsvu]

Sure, here's an example:

import rich.traceback
import multiprocessing


def fail():
    raise ValueError("Fail!")


if __name__ == "__main__":
    rich.traceback.install(suppress=[multiprocessing])
    with multiprocessing.Pool() as pool:
        pool.apply(fail)

Output with this PR:

Without this PR only the bottom red box with the multiprocessing traceback is printed. Note that it doesn't point to where the exception is actually thrown in the code.

[willmcgugan]

Can you look at the CI fails please.

[nilsvu]

Should be fixed, just needs your approval to run CI

[codecov-commenter]

Codecov Report

Merging #2397 (0817587) into master (19e518f) will decrease coverage by 0.03%. The diff coverage is 98.34%.

@@            Coverage Diff             @@
##           master    #2397      +/-   ##
==========================================
- Coverage   98.71%   98.67%   -0.04%     
==========================================
  Files          73       72       -1     
  Lines        7771     7765       -6     
==========================================
- Hits         7671     7662       -9     
- Misses        100      103       +3     

Flag	Coverage Δ
unittests	98.67% <98.34%> (-0.04%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
rich/default_styles.py	100.00% <ø> (ø)
rich/progress.py	92.76% <ø> (ø)
rich/segment.py	98.72% <93.10%> (+<0.01%)	:arrow_up:
rich/_inspect.py	100.00% <100.00%> (ø)
rich/box.py	100.00% <100.00%> (ø)
rich/cells.py	96.05% <100.00%> (-3.95%)	:arrow_down:
rich/console.py	98.30% <100.00%> (+0.01%)	:arrow_up:
rich/highlighter.py	100.00% <100.00%> (ø)
rich/pretty.py	99.22% <100.00%> (+<0.01%)	:arrow_up:
rich/style.py	99.75% <100.00%> (-0.01%)	:arrow_down:
... and 3 more

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[willmcgugan]

Thanks